The invention relates to a valve having particular application to the control of flow and pressure within a sanitary-sewer system.
Sanitary-sewer systems which operate in hilly terrain may include independently pumped feeders at elevations which are below an intervening region of greater elevation. For example, one or more septic-tank sources may discharge into a holding tank from which liquid-sewage is pumped into a feeder to the sewer line which runs up over a hill to serve one or more such feeders on the other side of the hill. It is customary to use one or more check valves to protect against back flow in that part of the sewer line (and its feeders) which climbs the hill in the downstream direction of liquid-sewage flow. And on the downslope after climbing the hill, it is customary to employ one or more relatively cumbersome relief valves (sometimes called pressure-sustaining valves) to assure against syphoning liquid sewage out of the greater-elevation region, as well as to assure against presentation of an excessive hydraulic head to septic-tank discharges into the sewer line below the relief-valve location. Each relief valve requires extreme care in establishing a solid local foundation from which a guide-axis of the valve can be fixed and assuredly vertical; heavy-weight elements are applied to the valve on the guide axis to preload the relief function to a level which will hopefully avoid syphoning. However, as a practical matter, the necessary field preparation of the local foundation by careless personnel often results in a guide axis that is not vertical, so that friction of the weight elements with respect to the guide axis becomes a serious factor in development of mechanical hysteresis in valve action; furthermore, there is a great inherent lack of precision involved in the preloading increments associated with use of such weight elements.